The present invention relates to a projection exposure method and apparatus which form a fine pattern such as an LSI pattern on a substrate by using a photomask and a projection lens.
In lithography using a projection exposure apparatus, fine patterns have been formed by decreasing the exposure wavelength and increasing the numerical aperture (NA) of a projection lens.
Although finer patterns can be formed with a decrease in wavelength and an increase in NA, the depth of focus tends to decrease as the patterns decrease in size. For this reason, with a decrease in the size of a pattern, the depth of focus of the optical image cannot meet the practical requirement.
FIG. 23 shows intensity distributions on an image projection plane on which projection images are projected when a projection system having an exposure wavelength .lambda. and a numerical aperture NA and using a lines and spaces pattern of 0.4 .lambda./NA in a normal projection exposure operation. Note that the lines and spaces pattern is a striped periodic pattern. In this case, lines are spaced 0.4.lambda./NA apart.
Referring to FIG. 23, a curve 231 represents the intensity distribution (continuous line) on the image projection plane when the focus position coincides with the image projection plane; a curve 232, the intensity distribution (dotted line) on the image projection plane when the focus position is shifted from the image projection plane by 0.52 .lambda./NA.sup.2 ; a curve 233, the intensity distribution (chain line) on the image projection plane when the focus position is shifted from the image projection plane by 0.50 .lambda./NA.sup.2 ; and a curve 234, the intensity distribution (chain double-dashed line) on the image projection plane when the focus position is shifted from the image projection plane by 1.00 .lambda./NA.sup.2.
In the intensity distribution 231 in a just focus state, i.e., when the image projection plane coincides with the focus position of the projection system, a lines and spaces pattern is formed. However, in the intensity distributions 232, 233, . . . in defocus states, the optical image of the pattern deteriorates. In the intensity distribution 233 with a defocus of .+-.0.5 .lambda./NA.sup.2, the intensity distribution of the image is flat and line patterns can not be created. For this reason, the optical depth of focus in the above normal projection exposure operation becomes .+-.0.5 .lambda./NA.sup.2.
For example, when NA=0.5 in a KrF exposure operation, the depth of focus is .+-.0.5 .mu.m, which is not a sufficient value in practice. Note that a KrF exposure operation is performed by using a KrF excimer laser having a wavelength of 0.248 .mu.m as a light source.
As described above, in a normal projection exposure operation, finer patterns cannot be formed without sacrificing practical depths of focus. This problem will be described below by exemplifying the lines and spaces pattern formed as a periodic pattern.
In such a normal projection exposure operation, an optical image is formed on an image projection plane when a plurality of diffracted light beams which have been generated on a photomask and passed through a projection lens interfere with each other. In this case, the smaller the pitch of the lines and spaces pattern on the photomask, the larger the angle at which light diffracted by the photomask spreads. If the pitch of the lines and spaces pattern is smaller as compared with the cutoff frequency, the angle defined by two adjacent diffracted light beams is large enough to go beyond the projection lens.
For this reason, with such a small pitch, only one of diffracted light beams generated on the photomask can pass through the projection lens. Since only one diffracted light beam reaches the image projection plane, no interference occurs on the image projection plane. No optical image, therefore, can be formed.
In some method, a projection lens having a large numerical aperture corresponding to the largest incidence angle of the lens is used to spread the range of the angle of the diffracted light beams. In another method, the exposure wavelength is decreased to decrease the angle of diffraction occurring on the photomask.
In either method, however, as a pattern to be formed is reduced to a submicron size (approaching the cutoff frequency of a currently available exposure optical system), the contrast deteriorates, and the depth of focus decreases. That is, both the methods are not practical.
In brief, in a normal projection exposure operation, finer patterns cannot be formed without sacrificing practical depths of focus. A demand, therefore, has arisen for a method which can form finer patterns without sacrificing the depths of focus.
As a method of forming finer patterns without sacrificing depths of focus, a method using a phase-shifting mask has been proposed (Levenson et al., IEEE Transactions on electron device vol. ED-29 (1982)). A method using modified illumination has also been proposed (Matsuo et al., IEDM (1991)).
In these methods, diffracted light generated in a direction parallel to the optical axis through a photomask is eliminated by using the phase of the light, thereby forming a fine periodic pattern with a practical depth of focus of 1 .mu.m or more.
As another method of forming a fine periodic pattern with a practical depth of focus of 1 .mu.m or more by eliminating diffracted light generated in a direction parallel to the optical axis through a photomask, a pupil filter method has been proposed (J. E. Jewell et al., SPIE Proceeding vol. 1088 Optical/Laser Microlithography II (1989), 496).
A multiple exposure method (Fukuda et al., IEEE electron device letter vol. EDL-8 (1987)) has also been proposed as a method of increasing the depth of focus, although this method cannot be used to reduce the size of a pattern.
The method using a phase-shifting mask will be described first.
FIG. 24 shows the arrangement of a phase-shifting mask. Reference numeral 241 denotes a substrate consisting of a transparent material such as quartz glass; 242, light-shielding portions each consisting of a Cr film formed on the surface of the substrate 241; 243, light-transmitting portions between the light-shielding portions 242; and 244, a shifter formed at every other light-transmitting portion 243.
In the method using the phase-shifting mask, as shown in FIG. 24, each shifter 244 is formed to make the respective adjacent light-transmitting portions 243 have a phase difference of 180.degree., thereby nullifying the intensity at the portions of the image projection plane which correspond to the light-shielding portions 242.
The method using this phase-shifting mask will be described from the viewpoint of diffraction of light.
FIGS. 25A, 25B, and 25C show arrangements for a projection exposure operation. Reference numeral 251 denotes illumination light; 251a, modified illumination light irradiated at a given incident angle; 252, a phase-shifting mask of the Levenson type; 252a, a general transmission mask; 253, 253A, and 253B, projection lenses; 254, an image projection plane; 255a and 255b, 1st- and -1st-order diffracted light beams; 256a and 256b, .+-.2nd-order light beams; 257a and 257b, .+-.3rd-order diffracted light beams; and 259, a pupil filter.
Referring to FIG. 25B, reference numeral 255c denotes a 0th-order light beam; 256c, a -1st-order diffracted light beam; and 257c, a -2nd-order diffracted light beam.
As shown in FIG. 25A, no diffracted light is generated by the phase-shifting mask 252 in a direction parallel to the optical axis.
This means that a 0th-order diffracted light beam parallel to the optical axis is eliminated upon interference of light on the mask surface, and only the .+-.1st-order diffracted light beams 255a and 255b generated in directions symmetrical with respect to the optical axis pass through the projection lens 253 to be used to form an optical image on the image projection plane 254.
With this operation, no diffracted light is generated in the optical axis direction by interference of light on the mask surface, but only diffracted light beams generated in the directions symmetrical with respect to the optical axis interfere with each other on the image projection plane, thereby forming an optical image. Diffracted light in the optical axis direction causes a deterioration in contrast owing to a defocus state.
In this case, the angle defined by the light beams interfering with each other on the image projection plane 254 can be set to the largest incident angle of the projection lens 253.
Under such conditions that cause light beams at the same angle with respect to the optical axis to interfere with each other, the intensity and shape of the optical image undergo no change even if the focus position is changed.
For this reason, the resolution limit can be set to the cutoff frequency of the projection system so that a fine lines and spaces pattern can be formed with a large depth of focus.
In this method, however, shifters for inverting the phase of transmitted light must be arranged on a phase-shifting mask, as shown in FIG. 24. For this reason, the number of steps in manufacturing a mask is larger than that in manufacturing a general transmission mask. In addition, since shifters consisting of a transparent material are arranged on the mask, inspection of defects in the mask and correction thereof are difficult to perform.
In contrast to this, in the method shown in FIG. 25B, which uses the modified illumination light 251a, the modified illumination light 251a is obliquely incident on the transmission mask 252a to make adjacent light-transmitting portions 261a of the conventional transmission mask 252a have a phase difference of 180.degree., thereby nullifying the intensity of at the portions of the image projection plane which correspond to light-shielding portions 261b, as shown in FIG. 26.
This method is especially effective for a periodic pattern, similar to the phase-shifting mask described above.
A pattern finer than that in the prior art can be formed with a sufficient depth of focus.
In addition, since a transmission mask made of a light-shielding member consisting of, e.g., Cr, which is used for a general exposure operation, is used, there is no difficulty in manufacturing a mask unlike the case wherein a phase-shifting mask is used.
FIG. 25B shows the directions of diffracted light beams in this method. These directions are the same as those in FIG. 25A.
In the method of using the modified illumination, as in the method using the phase-shifting mask, no diffracted light is generated in a direction parallel to the optical axis, and an image is formed by using diffracted light beams at the same angle with respect to the optical axis.
In this method, however, since the incident direction of illumination light has a close relationship with the repeating direction of a periodic pattern, the depths of focus of patterns in all directions cannot be increased.
As indicated by the perspective view of FIG. 27, the depth of focus of a pattern 271 on a mask 270 on the image projection plane can be increased by using illumination light 272 in the direction shown in FIG. 27.
The illumination light 272 is incident on a pattern 273, which is formed at 90.degree. with respect to the pattern 271, from a direction perpendicular to the repeating direction. For this reason, with regard to the pattern 273, no phase difference occurs between the adjacent light-transmitting portions, and hence no increase in depth of focus is attained.
As described above, the method using modified illumination exhibits a dependence on the periodicity direction of a periodic pattern. Annular illumination has been proposed to eliminate such dependence on the periodicity direction of a pattern.
However, this illumination shape is obtained by only averaging the direction dependencies of a pattern. For this reason, an increase in depth of focus in this method is smaller than that in the method using the optimal illumination shape. That is, a sufficient depth of focus cannot be obtained.
In the pupil filter method, as shown in FIG. 25C, the light-shielding filter (pupil filter) 259 is inserted in the pupil plane of the projection lenses 253A and 253B to block light diffracted by the photomask 252 in direction parallel to the optical axis. As a result, only .+-.1st-order diffracted light beams diffracted in directions symmetrical about the optical axis reach the image plane and interfere with each other. For this reason, a fine periodic pattern can be formed with a practical depth of focus as in the phase-shifting mask or modified illumination method.
In this method, however, since the light-shielding filter 259 is inserted in the pupil plane of the projection lens to block 0th-order diffracted light having a high intensity, which is transmitted through the mask without being scattered, heat generated by the light-shielding filter changes the magnification or aberration of the lens, or a problem is posed in terms of pattern design, that is, all areas on the mask on which no patterns are formed always become light-shielding areas.
A multiple exposure method (FLEX method) is also available, in which exposure is performed a plurality of number of times as the image projection plane position is shifted from the just focus position.
This method is especially effective for isolated patterns.
At an isolated pattern, the intensity decreases because of a defocus state. Consequently, if exposure is performed at a plurality of positions shifted from the just focus position, exposure at defocus positions hardly contributes to pattern formation because the optical image decreases in intensity.
In a multiple exposure operation, a pattern on a portion corresponding to an exposure operation at the focus position nearest to the just focus position is formed.
For this reason, pattern formation can be performed with a small defocus dependence.
In this multiple exposure method, however, the depth of focus is not theoretically increased infinitely, unlike in the above three methods.
In addition, since the intensity at each defocus position is added to the intensity at the just focus position, the contrast of the optical image deteriorates as compared with that of the optical image at the just focus position. For this reason, this multiple exposure method cannot cope with the formation of a fine pattern on the submicron order or less.
Furthermore, in the case of a periodic pattern such as a lines and spaces pattern, the exposure intensity does not decrease unlike in the case of an isolated pattern. For this reason, as shown in FIG. 23, only the contrast of the optical image deteriorates because of a defocus state.
When, therefore, exposure is performed at a plurality of positions shifted from the just focus position, images which are formed at the defocus positions and have poor contrast are superimposed on an image at the just focus position with almost equal intensities. That is, in the multiple exposure method, the above effect of increasing the depth of focus cannot be obtained.
With the above-described arrangement, according to the conventional method using a phase-shifting mask, it is not easy to manufacture a mask, forming shifters and the like.
In the method using modified illumination, a sufficient depth of focus cannot be obtained because of the direction dependence of a pattern and the like.
In the method using a pupil filter, heat is generated in the lens, and patterns to be formed are undesirably limited.
The multiple exposure method is not effective in reducing the size of a pattern, and offers no effect of increasing the depth of focus with respect to a periodic pattern.